Lift-generating rotor blades of a hingeless rotor for a rotorcraft are manufactured predominantly from fiber composite material. During continuous rotor operation, the rotor blades are deflected in various directions and heavily stressed as a result. The rotor blade usually possesses, at its end pointing toward a rotor head, a structural element having a flapwise-soft and lead-lag-soft region that allows motions of the rotor blade in a flapwise direction and a lead-lag direction. The structural element is also referred to as a flex beam. The flapwise-soft and/or lead-lag-soft region of the structural element is referred to overall as a flexurally soft region. In the direction of the longitudinal rotor-blade axis, the structural element is usually configured at its end pointing toward the rotor head with a blade connector that enables a connection to the rotor head or to a rotor-head plate.
The transition from the blade connector into the flexurally soft region is embodied to form a rotor-blade neck. The structural element on the one hand transfers drive torque from the rotor head to the rotor blade, and on the other hand transfers the centrifugal forces, acting on the rotor blade during rotor operation, to the rotor head. A disconnect point is often incorporated between the structural element and the rotor blade so that the structural element can be separately fabricated and more easily replaced in the event of damage. The lift-generating rotor blade region extends from this disconnect point to the outermost end of the rotor blade, i.e. to the rotor-blade tip.
The blade neck of the structural element generally possesses the flapwise-soft region, which constitutes a fictitious horizontally oriented axis (also called a fictitious or virtual flapping hinge) about which the rotor blade executes flapping motions. The distance between the fictitious flapping hinge and the rotor axis is referred to as the flapping hinge distance. In conventional hingeless rotors, the so-called fictitious or virtual flapping hinge distance is relatively large, constituting approx. 8 to 12% of the length of the rotor disc radius (measured from the rotor axis radially outward to the blade tip).
A large flapping hinge distance in a hingeless rotor results, during operation, on the one hand in good helicopter control response and maneuverability, but on the other hand, in particular, in a high natural flapping frequency. This relative high natural flapping frequency, and the vibrations that result therefrom in the case of a bearingless rotor, are disadvantageous in terms of the helicopter's flying characteristics, and lead to large stresses on the blade connector and blade neck. The blade connector and blade neck must therefore have correspondingly large dimensions in order to withstand the stress that occurs. In conventional helicopter rotors, a low natural flapping and lead-lag frequency is desirable for these reasons.
Because of the large stresses on the rotor blade and blade connector in a bearingless rotor, and the strength of those components that must be ensured in that context, it is extremely difficult to reduce the flapping hinge distance or decrease it below a specific value. In conventional bearingless rotors, a small flapping hinge distance would considerably reduce the durability and service life of the rotor blade in question, which is disadvantageous or even hazardous. On the other hand, however, a small flapping hinge distance would be desirable for a variety of applications, since helicopters having such a rotor are generally perceived by pilots, crew members, and passengers as being more comfortable.
A large flapping hinge distance can also be disadvantageous from an aerodynamic standpoint, since the total air resistance of the rotor elements extending from the rotor axis to the fictitious flapping hinge, in particular that of the aforesaid structural element, is quite high; and in addition, this region, which accounts for a relatively large proportion of the rotor radius, cannot be used for an aerodynamically effective region of the rotor blade.
DE 198 37 802 C1 discloses a hingeless rotor for a rotorcraft, encompassing a rotor head, a rotor mast having a rotor axis, a torque-transmission element nonrotatably joined to the rotor mast, at least one lift-generating rotor blade, and a rotor-head-side rotor-blade connector. In rotors of the aforesaid kind, the rotor-head-side rotor-blade connector usually encompasses, in addition to the structural element already described above, at least two bolts that are disposed substantially radially with respect to the rotation axis of the rotor and of the rotor blade. The flapping torque and lead-lag torque are transferred via these bolts. The structural element can be braced on the rotor-head plate by way of upper and lower support surfaces. The disadvantages explained previously exist with this design as well.
In conventional hingeless rotors for conventional helicopters, a low natural flap and lead-lag frequency is desirable, and is achieved by way of flapwise-soft and lead-lag-soft rotor-blade attachments. In special rotors, for example tilting rotors (also called tiltrotors) of tiltrotor helicopters or aircraft, however, a different design should be aimed at, for the following reasons: If the rotor is designed so that the natural lead-lag frequency of the rotor is less than the so-called excitation frequency, there is an elevated potential for the excitation of ground and air resonances. In conventional rotors, these resonance phenomena are controlled with dampers. The soft suspension of the tilting rotors on the wing of a tiltrotor helicopter, however, in contrast to a stiff cell of a conventional helicopter, causes undesired couplings between the natural wing frequency and the lead-lag frequency if the design falls below the excitation frequency. For these reasons, a more lead-lag-stiff rotor is necessary for tilting rotors. Conventional hingeless rotors are therefore not suitable for tiltrotor applications, and would result in strength and safety problems.